Radio frequency identification (RFID) systems are widely used for the identification and tracking of articles. An RFID tag may be affixed to an article for which tracking is desired. Examples of articles that may be tracked include an inventory item or a pallet on which inventory items are placed for storage or transport. The RFID tag may suitably include identification information, such an identification code that can be used to retrieve stored information. Alternatively, or in addition, the identification information may include actual descriptive information relating to the article. The tags are read with RFID readers, and noting the time of reading and location of the reader that is used to read a tag at a particular time may provide valuable information about the location and movement of articles.
An RFID reader reads a tag by transmitting an electromagnetic interrogation signal to the tag, and receiving a response from the tag. Depending on the design of the system, this response may be an active signal, in which the tag uses its own power source to generate a return signal, or a passive signal, in which the return signal is a reflection of the interrogation signal.
Significant advantages are achieved if an RFID reader has the capability of determining the distance to an RFID tag. Locating an article, or tracking movement of all article, is simplified by distance information. A distance measurement can be provided by interrogating a tag using two interrogation signals at different frequencies and using the frequency information and the phase difference information between the two signals to solve for distance.
However, the prevalence of reflected signals in the environment of the reader, conveniently referred to as static proximity reflected signals, tends to degrade ranging accuracy. In typical operation, the reader will encounter signals reflected from objects disposed around the reader, as well as the return signal from the tag. The objects disposed around the reader will typically be at different distances, and the phases of the reflected signals will therefore vary from one another and will also vary from the phase or phases of the return signals from the tag. The signals received by the reader will therefore include combined signals, each of which is a mixture of reflected signals and the direct signal from the tag. The signals will therefore include mixed phase information, rendering difficult or in some cases impossible the use of phase information to accurately compute distance.
Many desirable uses of a reader tend to increase the likelihood of receiving reflected signals. It is advantageous for the reader not to tightly focus the interrogation signals, because a broader focus for the signals allows for easier reading of a tag. If the signals are broadly focused, it is not necessary for a user to precisely direct an antenna at a tag. Instead, a reader may read tags that may be located at a range of angles from the reader. In addition, a broad focus allows for configurations such as a fixed tag reader detecting tags that come within range of the reader from a variety of directions. Such a configuration is particularly useful for readers that are installed at security checkpoints, because a thief trying to steal an object with an RFID tag attached is likely to attempt to avoid the signals emitted by a reader, and a broad focus for the signals makes such avoidance more difficult.